Traditionally, light-emitting diodes have been used mostly as indicator lights for a wide variety of devices. Due to advances in diode light intensity and color quality, modern LEDs are being used increasingly for area lighting applications, such as in indoor or outdoor lighting fixtures.
Reflectors/housings for LED and other lighting systems are typically metal or at least metal coated. Metal reflectors for lighting applications, most commonly incorporating aluminum and more rarely silver or other metals on the reflective surface, are widely used because their reflectivity is high, and largely insensitive to the incident angle of the light and they are amenable to the construction of a range of reflector shapes. Metals, though, exhibit limited reflectivity due to surface finish, degradation during processing (e.g. oxidation and surface contamination), and the application of protective coatings.
Other reflectors include a coating of thin films, such as polymeric or dielectric layers. The latter type can be referred to as a dielectric mirrors, or Bragg reflectors which use quarter wave designs A related term is a dichroic stack, mirror, or reflector, which has both quarter wave or non quarter wave designs
Polymer stacks have shortcomings including relatively-low robustness. Polymer stacks, being an example of non-dielectric stacks, can operate decently within only limited temperature range, for example. The polymer stacks can also break down when exposed to ultraviolet (UV) light, which can be generated by an LED or a high-intensity discharge (HID) light source. Non-dielectric stacks, e.g., polymer stacks, may also susceptible undesirably to water uptake and swelling.
Current reflective coatings for LED reflectors are typically vacuum coated aluminum. In theory, the reflectivity of vacuum coated reflectors is 91% in theory, but in practice is typically about 80%. As mentioned herein, silver can also be implemented. Silver provides improved performance for LED street lighting or parking lot lighting. Silver has much higher reflectivity than aluminum, typically about 98%. Silver has the drawback of requiring protection from tarnishing. Sulphides, water vapor, oxygen, carbinates in both molecular and atomic and ionic attack silver. Silver protection may be unreliable in that the protective layer involves multilayers and may not match the LED life.